Refiners used for refining fibrous material, such as wood chips, into pulp typically comprise one or more refiner elements positioned oppositely and rotating relative to each other. One or both of the refiner elements can be rotatable. A fixed i.e. stationary refiner element is called the stator and the rotating or rotatable refiner element is called the rotor. In disc refiners, the refiner elements are disc-like and in cone refiners the refiner elements are conical. In addition to disc refiners and cone refiners, there are also so-called disc-cone refiners where the material to be defibrated is first refined by disc-like refiner elements and then further refined between conical refiner elements. Furthermore, there are also cylindrical refiners where both the stator and the rotor of the refiner are cylindrical refiner elements.
The refiner elements are positioned such that a refining space/gap is formed between the inner surfaces, i.e. the surfaces opposing one another, of the refiner segments. In disc refiners, which represent the most common refiner type, the material to be refined is usually fed through an opening in the middle of one of the refiner discs, usually the stator, to a central space between the discs. The material is then forced by the centrifugal force towards the circumference of the discs to emerge in the refining space/gap, where the refining/grinding of the fibrous material is carried out. The refined material is discharged from the refining space/gap, from the outer periphery of the refining surfaces of the refiner discs, to be fed onwards in the pulp manufacturing process.
The inner (refining) surfaces of the refiner elements are typically provided with one or more refiner segments, which are formed with a pattern of bars and intermediate grooves of different sizes and orientations, for improving the grinding action on the fibers. The refiner segments are typically positioned adjacently in such a way that each refiner segment forms part of a continuous refining surface. The pattern of bars and grooves may be divided into different zones located outside each other, e.g. a radially inner inlet zone where the fibrous material is fed into the refiner, and a radially outer refining zone where the refining of the material takes place. In the inlet zone there are usually fewer bars and grooves, and the pattern is coarser than in the refining zone. Normally, the bars and grooves of the refiner segments extend substantially radially with respect to the rotational center of the refiner elements.
When the fibrous material is refined in the refining space/gap between the refiner elements, some of the moisture in the material is turned into steam. The steam flow is usually very irregular, but some steam will flow towards the circumference of the refiner elements along with the material, and some of the steam will also flow “backwards” towards the center of the refiner elements. The steam flow will depend—among other things—on how the refiner segments are designed. The back-streaming steam will mainly flow in the grooves formed between the bars of the refiner segments towards the center of the refiner elements.
Usually, flow restrictions or dams are inserted in the grooves in the refiner segments in order to prevent unprocessed material to pass out through the refining gap. The dams guide the material to the space between opposite refiner bars, and thereby refining of the material can be promoted. However, the dams constitute an obstacle to the steam developed in the refining gap during the refining process. The steam is also forced upwards out of the grooves by the dams and disturbs the material flow through the refining gap.
This in turn leads to blockage on the refining surface, which may affect the stability of the refining gap, rendering the material flow through the gap non-uniform. Variations in feed within the refining gap causes a decrease in the production capacity of the refiner, non-uniformity of the quality of the refined material and an increase in the energy consumed for the refining. Therefore, there is a need for improving the design of the refiner segments in order to overcome the above mentioned disadvantages.